The present invention relates to a device for the highly accurate adjustment of two parts which are in play-free threaded engagement, such as for instance the mounts of optical elements which are displaced for purposes of focusing.
Such threads are, in general, made free of play by being clamped axially against each other, for instance by means of springs; this measure is sufficient in most cases, particularly when threads of small diameter are concerned and no extreme demands are made on the accuracy of positioning and centering.
With an increase in the diameter of the thread and increasing demands on the precision of centering, the friction in the thread, however, increases to such an extent that adjustment is possible only with the expenditure of unreasonably great force. In the case of spring-clamped thread parts there is, furthermore, always the danger that, in the presence of strong vibration, forces will temporarily occur which impermissibly reduce or even entirely neutralize the spring-clamping force. The position of parts fastened to the thread is then no longer maintained with the required precision, which, in the case of optical parts, means a defocusing of the system.
A further possibility of reducing thread play in the prior art consists in individually grinding the threaded parts with polishing agents. Aside from the rather high manufacturing expense involved in this method, the parts to be ground must consist of different materials in order to prevent their "seizure" in later operation. However, since different materials have different coefficients of thermal expansion, the temperature range within which such threads are free of play is very greatly limited, particularly in the case of large diameters.
Threads are known which are made free of play by balls inserted under compression between rotatable parts. However, such ball-rotation spindles considerably increase the size of the thread. Furthermore, due to the fact that the balls must have a given minimum diameter to sustain surface pressure under load, the pitch of thread advance cannot be reduced, at will, to the value required, for instance, for fine threads, so that ball-thread applications are limited.